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Tips & Tricks: 3-2-1 Jig Theory applied to FEA Boundary Conditions

Any solid body as 6 rigid body degrees of freedom, as shown in the figure below. The goal of applying good boundary conditions in most Finite Element Analyses (FEA's) is to restrain these translations and rotations so that it can still deform in any direction without stiffening the part in any way.

If the applied constraints are just sufficient to keep the part in equilibrium and no more, this will usually provide conservative stresses and deflections. Note that most free or express versions of simulation software force over-constraint of your models, and so can be unconservative (very dangerous!)

6dof2
6 rigid body degrees of freedom
Jig and Fixture design is also about restraining rigid body degrees of freedom without inducing deflections into the part. The 3-2-1 theory of fixture design is illustrated below:

  1. First rest the work piece on three non-collinear points. This will fix the translation in the z-direction and two rotations about the x-axis and y-axis.
  2. Then slide the part to two points to fix the translation in the y-direction and the rotation about the z-axis.
  3. Finally slide the part to one point to fix the translation in the x-direction.
321

This restrains all 6 rigid body degrees of freedom with the minimum number of boundary conditions. Applying this jigging method to FEA results in constraining one point in all three translational freedoms, a second point in twotranslational freedoms and a third in one translational freedom.

fea2

To check that the loads are balanced and that the boundary conditions are correct, simply check that they carry no load. You can read the rest of the article here.

Don't hesitate to contact us if you need some tricky boundary conditions analysed!

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